Quantitative information on regional cropland runoff is important for sustainable agricultural water quantity and quality management. This study combined the Soil Conservation Service Curve Number (SCS-CN) method and geostatistical approaches to quantify long-term (1990–2013) changes and regional spatial variations of cropland runoff in China. Estimated CN values from 17 cropland study sites across China showed reasonable agreement with default values from the National Engineering Handbook (R2 = 0.76, n = 17). Among four commonly used geostatistical interpolation methods, the inverse distance weighting (IDW) method achieved the highest accuracy (R2 = 0.67, n = 209) for prediction of cropland runoff. Using default CN values and the IDW method, estimated national annual cropland runoff volume and runoff depth in 1990–2013 were 253 ± 25 km3 yr−1 and 182 ± 15 mm yr−1, respectively. Estimated cropland runoff depth gradually increased from the drier northwest inland region to the wetter southeast coastal region (range: 2–1375 mm yr−1). Regionally, eastern, central and southern China accounted for 39% of the cultivated area and 53% of the irrigated land area and contributed to 68% of the national cropland runoff volume. In contrast, northwestern, northern, southwestern and northeastern China accounted for 61% of the cultivated area and 47% of the irrigated land area and contributed to 32% of the runoff volume. Rainfall was the main source (72%) of cropland runoff for the entire nation, while irrigation became the main source of cropland runoff in drier regions (northwestern and southwestern China). Over the 24-year study period, estimated cropland runoff depth showed no significant trends, whereas cropland runoff volume and irrigation-contributed percentages decreased by 7% and 35%, respectively, owing to implementation of water-saving irrigation technologies. To reduce excessive runoff and increase water utilization efficiencies, regionally specific water management strategies should be further promoted. As the first long-term national estimate of cropland runoff in China, this study provides a simple framework for estimating regional cropland runoff depth and volume, providing critical information for guiding developments of management practices to mitigate agricultural nonpoint source pollution, soil erosion and water scarcity.
Global nitrogen (N) and phosphorus (P) transport models provide a quantitative assessment of riverine nutrient source, transport, accumulation and depletion processes. By conducting a meta-analysis of the modeled results and accuracy metrics from previous studies, this study evaluated the accuracies and their influencing factors of three prevalent global riverine N and P transport models (Global NEWS, IMAGE-GNM and WorldQual). The Global NEWS model exhibited higher accuracies in predicting riverine dissolved organic nitrogen (DON; R2 = 0.58, NSE = 0.57) and dissolved organic phosphorus (DOP; R2 = 0.59, NSE = 0.59) yields compared to riverine dissolved inorganic nitrogen (DIN; R2 = 0.56, NSE=-0.80) and dissolved inorganic phosphorus (DIP; R2 = 0.33, NSE=-0.12) yields. The DIN and DIP sub-models of Global NEWS were applicable for basins with areas greater than 2.2×104 km2 and 3.2×104 km2, respectively. The IMAGE-GNM model demonstrated satisfactory accuracies in predicting riverine total nitrogen (TN; R2 = 0.56, NSE = 0.53) and total phosphorus (TP; R2 = 0.59, NSE = 0.48) concentrations, particularly in European basins. The IMAGE-GNM model performed better for simulation of riverine TN concentration when data set was longer than 21 years and for regions above 54°N, and for simulation of riverine TP concentration when data set was longer than 22 years and for regions above 55°N. The WorldQual model demonstrated relatively poor performance in simulating riverine TN (R2 = 0.76, NSE = 0.34) and TP (R2 = 0.71, NSE=-0.25) concentrations. For model improvements in future, the Global NEWS and WorldQual would benefit from more detailed in-stream nutrient retention/release and transformation modules, while improved chemical weathering dynamics could further enhance the Global NEWS. For the IMAGE-GNM, modification of the soil erosion module is warranted to enhance efficiency in basins outside Europe. Consideration of legacy effects is required to improve these three models. The results of this study provide valuable guidance for the model selecting and improvement for specific needs.
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